L07- GI development and embry Flashcards
Development of the gut tube
1) Starting the 3rd week of development, the lateral plate mesoderm layer splits into parietal and visceral layers. Parietal layer is paired with ectoderm (together known as somatopleure) whereas visceral layer is paired with endoderm (together known as splanchnopleure) . Both layers are continuous with each other.
2) The trilaminar embryonic disc then starts to roll up into a tube during which both layers grow laterally and fold ventrally. Somatopleure becomes the ventral body wall and splanchnopleure becomes the gut tube. The space between them is the primitive body cavity.
3) At the same time, the growth of the head and tail region of the embryo causes it to curve into the fetal position. Closure of the ventral body wall is complete by the 4th week of development except at the region of the connecting stalk where the mid gut communicates with the yolk sac through vitelline duct. The duct and the yolk sac normally degenerate by about 3 months of gestation.
splanchnopleure layers and gut tube components
Endoderm forms the epithelial lining whereas mesoderm forms the muscles, connective tissues and peritoneal components of the gut wall.
Primitive gut portions
Foregut: From pharynx to liver bud (esophagus, stomach, proximal duodenum)
Midgut: From caudal of liver bud to the junction between the right two- thirds and left one-third of the transverse colon (distal duodenum, jejuneum, ileum, ascending colon, proximal transverse colon)
Hindgut: From the left third of the transverse colon to the cloacal membrane (dital transverse colon, descending colon, sigmoid colon, rectum, anal canal)
Septum transversum
- The septum transversum arises from the mesoderm surrounding the heart between the primitive thorax and stalk of the yolk sac.
- Contributes to the formation of diaphragm, liver (connective tissues), and ventral mesenteries (terminal part of esophagus, the stomach, and proximal portion of the duodenum)
Diaphragm formation
Septum transversum, pleuroperitoneal membrane and muscular ingrowth from para-axial mesoderm together forms the diaphragm
Esophagus development
1) At the 4th week of development, the respiratory diverticulum appears at the ventral wall of the foregut.
2) Soon the tracheoesophageal septum separates the diverticulum from the foregut. The ventral portion of the foregut in the throax therefore becomes the trachea and lung buds whereas the dorsal portion becomes the esophagus.
3) Initially esophagus is short but it lengthens rapidly. The proliferation of esophageal epithelium almost obliterates the lumen, but the recanalization of the esophagus occurs by the end of the 8th week.
Stomach development
1) The stomach appears as a fusiform dilatation of the foregut during the 4th week of development
2) It rotates 90 degrees clockwise about the longitudinal axis such that the left side faces anteriorly. [Hence the left vagus nerve innervates the anterior aspect of the stomach and the right nerve innervating the posterior aspect]
3) The original anterior wall becomes the lesser curvature and the original posterior wall grows more quickly and therefore forming the greater curvature.
4) the stomach also rotates for 90 degrees clockwise about the anteroposterior axis resulting in the caudal part moving to the right and upward and the cranial part moving to the left and downward.
Innervation of stomach walls
The left vagus nerve innervates the anterior aspect of the stomach and the right nerve innervating the posterior aspect.
This is because the stomach rotates 90 degrees clockwise about the longitudinal axis such that the left side faces anteriorly and the right side faces posteriorly.
Development of duodenum
1) The duodenum derives from the terminal portion of the foregut and the proximnal portion of the midgut.
2) As the stomach rotates the duodenum takes on the form of a C-shaped loop and rotates to the right.
3) Duodenum is secondarily retroperitoneal except the very proximal portion (which is intraperitoneal) close to the pylorus of the stomach.
Liver development (and biliary system)
1) The liver bud arises as a ventral outgrowth from the endodermal epithelium at the distal end of the foregut at the 4th week of development.
2) It soon extends towards and penetrates the septum transversum
–> The endodermal lining of the foregut differentiates into the liver cells (hepatocytes) and the epithelial lining of the biliary system,
–> other cell types of the liver (Kupffer cells and hematopoietic cells) and the connective tissues are derived from the mesoderm of the septum transversum.
3) The connection between the liver bud and the foregut narrows to form the biliary ducts and a small outgrowth from the biliary duct forms the gallbladder.
Development of pancreas
1) Pancreas is formed by the dorsal and ventral buds which originate from the endodermal lining of the foregut (proximal duodenum)
2) As the duodenum rotates to the right and becomes C-shaped, the ventral bud moves dorsally (together with the entrance of the common bile duct into the duodenum) and comes to lie below and behind the dorsal bud.
3) The ventral bud forms the uncinate process and inferior part of the head whereas the dorsal bud forms the remaining part of the gland.
4) Hormone secreting cells and epithelial lining of the gland are derived from the endodermal lining of the foregut whereas the connective tissues are derived from the mesoderm surrounding the gland.
Foregut mesentery development
1) Initially, the whole gut tube and its derivatives are suspended from the dorsal body wall by dorsal mesenteries (intraperitoneal organs). The dorsal mesenteries contain vessels and nerves to and from the abdominal viscera.
2) Some organs are even connected to the ventral body wall by ventral mesenteries. These organs are the terminal part of esophagus, the stomach, and proximal portion of the duodenum. Their ventral mesenteries are derived from the septum transversum.
At first, the stomach is attached to the dorsal body wall by dorsal mesentery and to the ventral body wall by ventral mesentery.
3) As the stomach rotates, the ventral mesentery is pulled to the right and the dorsal mesentery pulled to the left.
4) At about the 5th week of development, primitive spleen starts to develop between the two leaves of dorsal mesentery. The dorsal mesentery also invests the pancreas as it develops.
5) The mesentery between the stomach and the spleen is known as gastrolienal/gastrosplenic ligament. As soon as the pancreas becomes secondarily retroperitoneal, the spleen, is connected to the body wall in close proximity to the left kidney and therefore this part of the dorsal mesentery is known as lienorenal/splenorenal ligament.
6) As a result of the rotation of the stomach about the anteroposterior axis, the dorsal mesentery of the stomach bulges down and continues to extend downwards as a four–layered greater omentum - These four layers fuse together and also fuse with the mesentery of the transverse colon hanging from the greater curvature of the stomach.
7) As soon as the liver bud grows towards the septum transversum, the ventral mesentery of the foregut further develops into the lesser omentum and the falciform ligament. The falciform ligament extend from the liver to the ventral body wall and the lesser omentum extend from the stomach and upper duodenum to the liver.
8) Interestingly, the posterior leaf of the dorsal mesenteries and the peritoneum of the posterior body wall degenerate for some organs (which originate from the gut tube) rendering them retroperitoneal. These organs are known as secondarily retroperitoneal (because they are first intraperitoneal then eventually become retroperitoneal). These organs include the pancreas, the duodenum (except the proximal portion), ascending colon, descending colon, and sigmoid colon.
Common congenital disorders of the esophagus
1) Oesophageal atresia and tracheo- esophageal fistula
Cause: Posterior deviation of the tracheoesophageal septum
Feature: Normal passage of amniotic fluid into the intestinal tract is prevented resulting in accumulation of excess fluid in the amniotic cavity (polyhydramnios)
2) Isolated oesophageal atresia
Cause: Failure to recanalise during the 8th week
Feature: much less common
3) Isolated tracheo- esophageal fistula
Cause: Failure of the tracheo- esophageal septum to divide the ventral and dorsal portions
Feature: much less common
Congenital disorder of stomach
Pyloric stenosis
Cause: Hypertrophy of muscles
Features: Presents with projectile vomiting few days after birth; More common in males
Congenital disorder of duodenum
Duodenal Atresia (complete occlusion); less severe form: duodenal stenosis (incomplete occlusion)
Cause: Failure to recanalise the lumen (normal development requires recanalization of the lumen which is initially occluded by epithelial cells)
Features:
- Usaully occurs at the hepatopancreatic ampulla
- Characterised by biliary vomiting
- One third have Down’s syndrome
Development of Midgut
1) There is temporary connection with the yolk sac through the vitelline duct until the 3rd month of development.
2) Entirely supplied by superior mesenteric artery, it develops into the distal part of duodenum, jejunum, ileum, caecum, appendix, ascending colon and proximal two-thirds of the transverse colon.
3) Because of the rapid growth of the midgut, the gut normally herniates into the umbilical cord during the 6th week. The midgut rotates anticlockwise for 90 degrees about the axis of the superior mesenteric artery during herniation.
4) The herniated part of the midgut returns to the abdominal cavity by about the 10th week of development. And now the gut makes another 180 degrees anticlockwise rotation when the herniated part returns to the abdominal cavity.
5) The caecal bud is the last part to return to the abdominal cavity. It appears in the 6th week and first lies in the right upper quadrant. But then it descends into right iliac fossa. The appendix forms from a diverticulum in the distal end of the caecal bud.
Omphalocele
Omphalocele
Cause: Congenital midgut disorder. Failure of the herniated gut to return to the abdominal cavity
Features:
- Covered by amnion
- Associated with other malformation and/or chromosomal disorders
Meckel’s diverticulum
Meckel’s diverticulum
Cause: Congenital midgut disorder. Persistent remnant of vitelline duct
Features:
- Antimesenteric
- 2% incidence, 2 inches long, two feet from the ileocaecal junction
- may get infected
Development of Hindgut
1) The hindgut gives rise to the distal third of transverse colon, descending colon, sigmoid colon, rectum and upper part of the anal canal.
2) The endoderm of the hindgut also forms the epithelial lining of the bladder and uretha.
3) The distal part of the hindgut is dilated and known as the cloaca, which terminates as the cloacal membrane.
4) Ventrally the cloaca also opens to the allantois in the connecting stalk, which serves as the exchange of excretory or waste products from the embryo.
5) The cloaca is divided into the anterior portion (urogenital sinus) and posterior portion (anal canal) by the urorectal septum (mesodermal), which starts to develop at the 4th week at the angle between the allantois and the hindgut towards the cloacal membrane.
6) The upper part of the anal canal is formed by endoderm of the hindgut and the lower part derives from invagination of the ectoderm.
7) At the end of the 7th week, the cloacal membrane ruptures to establish the continuity between the upper and lower parts of the anal canal.
8) The urogenital sinus will develop into the urinary bladder and the urethra.
Rectovaginal/ rectovesical/ rectourethral fistula
Rectum connected to abnormal sites such as vagina, urethra, bladder, but not anus
Cause: Failure to urogenital septum to develop caudually
Feature: Associated with anorectal agenesis
